1. Field of the Invention
The invention relates to a lifting clamp for clamping a workpiece to be lifted.
2. Description of the Related Art
Such a lifting clamp is disclosed in, inter alia, U.S. Pat. No. 761,282 and U.S. Pat. No. 3,851,358, and the applicant has been putting it on the market for years now. For the horizontal lifting of, for example, metal plates a plurality of lifting clamps at a distance from each other on opposite sides are generally used, as shown, for example, in FIG. 1 of U.S. Pat. No. 815,452.
Furthermore, U.S. Pat. No. 4,595,225 discloses a lifting clamp which is provided with an electromagnet for the purpose of keeping a clamping part open. A problem with this clamp is the positioning of the clamp before the lifting operation. The clamp will grip a workpiece only at the time of lifting; before that, the clamp can fall off or become detached from the workpiece.
The positioning of such lifting clamps, which for heavy workpieces are often made heavy themselves, often cannot be performed by one person. In addition, the lifting clamps can slip off before the start of pulling on the lifting eyes. A clamping arm is often pretensioned by means of a spring or the like in the clamping device. However, in order to ensure reliable clamping, this pretensioning must be considerable, and in the case of large clamps a spring is not sufficient. This means that a lifting clamp is generally made heavier than is strictly necessary for strength and/or a bearing part is long and difficult to handle. There is therefore scope for improvement on these lifting clamps which have been known per se for a long time now.